|Cbl-b is a critical regulator of macrophage activation associated with obesity-induced insulin resistance in mice.|
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|PMID: 23349502 Owner: NLM Status: MEDLINE|
|We previously reported the potential involvement of casitas B-cell lymphoma-b (Cbl-b) in aging-related murine insulin resistance. Because obesity also induces macrophage recruitment into adipose tissue, we elucidated here the role of Cbl-b in obesity-related insulin resistance. Cbl-b(+/+) and Cbl-b(-/-) mice were fed a high-fat diet (HFD) and then examined for obesity-related changes in insulin signaling. The HFD caused recruitment of macrophages into adipose tissue and increased inflammatory reaction in Cbl-b(-/-) compared with Cbl-b(+/+) mice. Peritoneal macrophages from Cbl-b(-/-) mice and Cbl-b-overexpressing RAW264.7 macrophages were used to examine the direct effect of saturated fatty acids (FAs) on macrophage activation. In macrophages, Cbl-b suppressed saturated FA-induced Toll-like receptor 4 (TLR4) signaling by ubiquitination and degradation of TLR4. The physiological role of Cbl-b in vivo was also examined by bone marrow transplantation and Eritoran, a TLR4 antagonist. Hematopoietic cell-specific depletion of the Cbl-b gene induced disturbed responses on insulin and glucose tolerance tests. Blockade of TLR4 signaling by Eritoran reduced fasting blood glucose and serum interleukin-6 levels in obese Cbl-b(-/-) mice. These results suggest that Cbl-b deficiency could exaggerate HFD-induced insulin resistance through saturated FA-mediated macrophage activation. Therefore, inhibition of TLR4 signaling is an attractive therapeutic strategy for treatment of obesity-related insulin resistance.|
|Tomoki Abe; Katsuya Hirasaka; Sachiko Kagawa; Shohei Kohno; Arisa Ochi; Kenro Utsunomiya; Atsuko Sakai; Ayako Ohno; Shigetada Teshima-Kondo; Yuushi Okumura; Motoko Oarada; Yoichi Maekawa; Junji Terao; Edward M Mills; Takeshi Nikawa|
|Type: Journal Article; Research Support, Non-U.S. Gov't Date: 2013-01-24|
|Title: Diabetes Volume: 62 ISSN: 1939-327X ISO Abbreviation: Diabetes Publication Date: 2013 Jun|
|Created Date: 2013-05-24 Completed Date: 2013-08-02 Revised Date: 2014-06-03|
Medline Journal Info:
|Nlm Unique ID: 0372763 Medline TA: Diabetes Country: United States|
|Languages: eng Pagination: 1957-69 Citation Subset: AIM; IM|
|APA/MLA Format Download EndNote Download BibTex|
Adaptor Proteins, Signal Transducing
Insulin Resistance / genetics, physiology
Macrophage Activation / genetics, physiology
NF-kappa B / metabolism
Obesity / genetics, metabolism*, physiopathology*
Proto-Oncogene Proteins c-cbl / genetics, metabolism*
Real-Time Polymerase Chain Reaction
|R01 DK089224/DK/NIDDK NIH HHS|
|0/Adaptor Proteins, Signal Transducing; 0/NF-kappa B; EC 6.3.2.-/Proto-Oncogene Proteins c-cbl; EC 184.108.40.206/Cblb protein, mouse|
Journal ID (nlm-ta): Diabetes
Journal ID (iso-abbrev): Diabetes
Journal ID (hwp): diabetes
Journal ID (pmc): diabetes
Journal ID (publisher-id): Diabetes
Publisher: American Diabetes Association
© 2013 by the American Diabetes Association.
Received Day: 25 Month: 5 Year: 2012
Accepted Day: 17 Month: 1 Year: 2013
Print publication date: Month: 6 Year: 2013
Electronic publication date: Day: 20 Month: 5 Year: 2013
Volume: 62 Issue: 6
First Page: 1957 Last Page: 1969
PubMed Id: 23349502
Publisher Id: 0677
|Cbl-b Is a Critical Regulator of Macrophage Activation Associated With Obesity-Induced Insulin Resistance in Mice|
|Edward M. Mills5|
1Department of Nutritional Physiology, Institute of Health Biosciences, The University of Tokushima Graduate School, Tokushima, Japan
2Medical Mycology Research Center, The University of Chiba, Chiba, Japan
3Department of Immunology and Parasitology, Institute of Health Biosciences, The University of Tokushima Graduate School, Tokushima, Japan
4Department of Food Science, Institute of Health Biosciences, The University of Tokushima Graduate School, Tokushima, Japan
5Pharmacology/Toxicology, College of Pharmacy, University of Texas at Austin, Austin, Texas
|Correspondence: Corresponding author: Takeshi Nikawa, firstname.lastname@example.org.
Obesity is sometimes associated with a chronic inflammatory state, insulin resistance, and type 2 diabetes (1,2). One feature of obesity-induced inflammation is the recruitment of macrophages into white adipose tissue (WAT). These activated macrophages secrete proinflammatory cytokines, such as tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), and monocyte chemoattractant protein-1 (MCP-1), and induce localized insulin resistance (3–5). Furthermore, the cytokines, adipokines, and free fatty acids (FAs) released from the WAT also act on the endocrine system, liver, and muscles and may reduce insulin sensitivity within these organs (6).
Casitas B-cell lymphoma-b (Cbl-b) is a unique ubiquitin ligase involved in the maturation and activation of macrophages and T lymphocytes (7,8). Expression of Cbl-b is upregulated with macrophage differentiation of the HL60 and U937 cell lines (7). We have already reported that recruitment of macrophages into the WAT is markedly augmented in elderly Cbl-b–deficient (Cbl-b−/−) mice and that their accumulation is associated with systemic insulin resistance and glucose intolerance (9). The current study demonstrated that a high-fat diet (HFD) induced accumulation of macrophages in WAT and insulin resistance in Cbl-b−/− mice at an earlier stage than in Cbl-b−/− mice fed a normal diet (ND). Feeding an HFD for 5 weeks induces macrophage accumulation and insulin resistance even in young Cbl-b−/− mice, whereas at least a 20-week period is often required to induce a similar phenomenon in Cbl-b−/− mice fed an ND (9).
Free FAs, especially saturated FAs, induce activation of macrophages and aggravate the inflammatory process in WAT (6,10,11). Saturated FAs can activate Toll-like receptor 4 (TLR4), which is a receptor for lipopolysaccharides (LPS), and trigger the activation of its adaptor protein myeloid differentiation factor 88 (MyD88) and a downstream signaling cascade, leading to upregulation of cytokine expression through the nuclear factor-κB (NF-κB) inflammatory signaling pathway (12–14). Recent experiments showed that Cbl-b binds to TLR4 and MyD88 in neutrophils and TLR4-overexpressing HEK293 cells (15). We therefore hypothesized that Cbl-b is a critical regulator of saturated FA-induced cytokine expression in macrophages and neutrophils.
The aim of this study was to elucidate the pathophysiological role of Cbl-b in the mechanism of macrophage activation by saturated FAs. The results showed that Cbl-b deficiency was associated with recruitment of macrophages into the WAT at the early stages of HFD-induced obesity. Cbl-b is an important molecule in diet-induced obesity and insulin resistance, especially through the activation of macrophages.
Cbl-b−/− mice were provided by the National Institutes of Health (Rockville, MD) and were backcrossed more than eight times into the C57BL/6 strain (16,17). We used C57BL/6 mice (Japan SLC, Shizuoka, Japan) as wild-type (Cbl-b+/+) mice.
Eight-week-old Cbl-b+/+ and Cbl-b−/− mice were divided into two groups; the first group was fed an ND (10% of the calories as fat), whereas the mice of the second group were fed an HFD (60% of the calories as fat). Eritoran is an analog of LPS that antagonizes its activity by binding to the TLR4–MD-2 complex (18). Eritoran and its placebo were a gift from Eisai Research Institute (Andover, MA). After being fed the HFD for 5 weeks, Cbl-b+/+ and Cbl-b−/− mice were injected intraperitoneally with Eritoran (4 mg/kg body weight) or placebo, three times every 24 h. The experimental protocols described in this study were approved by the Tokushima University Ethics Review Committee for Animal Experimentation.
Isolation of stromal vascular (SV) fraction and flow cytometry was conducted as described in detail previously (9). To determine cell-surface TLR4 protein levels in peritoneal macrophages from Cbl-b+/+ (n = 3) and Cbl-b−/− mice (n = 3), these macrophages were incubated with fluorescein isothiocyanate isomer-I–labeled anti-F4/80 (eBioscience, San Diego, CA) and phycoerythrin-labeled anti-TLR4 (eBioscience) antibody.
Bone marrow transplantation (BMT) was conducted as described previously (19). Bone marrow cells (5 × 106 cells) were harvested from Cbl-b+/+ and Cbl-b−/− mice by flushing with PBS. The recipient 5-week-old C57BL/6 mice were lethally irradiated at a dose of 7 Gy with an irradiator (MBR-1520R-3, Hitachi, Tokyo). Bone marrow cells were then injected in the retro-orbital venous plexus of the recipient mice. The mice were fed the HFD 3 weeks later. We confirmed the efficiency of this BMT technique by measuring the ratio of bone marrow cells derived from green fluorescence protein (GFP) transgenic mice to the bone marrow of wild-type mice. The proportion of GFP-positive cells at 8 weeks after BMT, as analyzed by flow cytometry, was 91.7 ± 3.3% relative to the total bone marrow cells in recipient mice.
Paraffin-embedded sections of epididymal WAT (6-μm thickness) were prepared and stained by standard techniques for histological analysis (9).
The RAW264.7 macrophage cell line (ATCC, Rockville, MD) was maintained in Dulbecco’s modified Eagle’s medium (DMEM) containing 10% FBS, 100 U/mL penicillin, and 100 µg/mL streptomycin at 37°C under a 5% carbon dioxide-95% air mixture. HEK293-mTLR4/MD2-CD14 (HEK293/TLR4) cells (InvivoGen, San Diego, CA), an isolated clone of HEK293 cells stably transfected with mouse TLR4, MD2, and CD14 genes, were maintained with DMEM containing 10 μg/mL blasticidin (InvivoGen) and 50 μg/mL high-purity hygromycin B (InvivoGen).
RAW264.7 macrophages and HEK293 cells were transfected with plasmid vectors containing the indicated genes by using a FuGENE HD lipofection reagent (Roche Diagnostics, Tokyo, Japan). The expression plasmids used in this study were prepared as described previously (17): pcDNA3.1-TLR4-V5, pcDNA3.1-MyD88-Myc, pCEFL-Cbl-b–hemagglutinin (HA), and pcDNA3.1-FLAG-ubiquitin. Mock vector, such as pCEFL, was used as the control vector.
Peritoneal macrophages, RAW264.7 cells, and HEK293 cells were treated with 400 μmol/L palmitate (C16:0) conjugated with BSA. Peritoneal macrophages were also pretreated with chemical inhibitors or a neutralizing antibody 1 or 2 h before palmitate treatment, respectively: 500 nmol/L inhibitor of NF-κB kinase (IKK) 2-IV inhibitor (Calbiochem) as NF-κB inhibitor, 25 μmol/L SP600125 (Calbiochem, San Diego, CA) as Jun N-terminal kinase (JNK) inhibitor, and 18 μg/mL of anti-TLR4/MD-2 IgG (eBioscience, San Diego, CA). We checked the efficacy of this neutralizing antibody against LPS-mediated IL-6 expression in peritoneal macrophages (Supplementary Fig. 1).
Immunoblot and immunoprecipitation analyses were performed as described previously (20). The following antibodies were used: anti–β-actin (Oncogene Research Products, San Diego, CA), anti-Akt1 (PharMingen International, Tokyo), antiphospho-Akt, antiphospho-inhibitor of NF-κB (IκB)-α, anti-JNK, antiphospho-JNK, anti-Myc (Cell Signaling Technology, Beverly, MA), anti-HA.11 (BabCo, Richmond, CA), anti-V5 (Invitrogen), anti–Cbl-b (Santa Cruz Biotechnology, Santa Cruz, CA), anti-FLAG M2 (Sigma), anti-MyD88 (Biovision, Milpitas, CA), anti-TRIF (Novus Biologicals, Littleton, CO), and anti–IκB-α (Upstate Biotechnology, Lake Placid, NY).
Real-time RT-PCR was performed with SYBR green dye using the ABI 7300 real-time RT-PCR system (Applied Biosystems, Foster City, CA), as described previously (21). The oligonucleotide primers used for amplification are listed in Supplementary Table 1.
The protein concentration was determined by bicinchoninic acid assay, as described previously (22). Serum concentrations of triglyceride and nonesterified FAs were measured with respective kits, as described previously (23,24). The concentrations of serum insulin, leptin, adiponectin, and MCP-1 were determined by using enzyme-linked immunosorbent assay (ELISA) kits (Morinaga Institute, Tokyo; Otsuka, Tokyo, Japan and R&D Systems, Minneapolis, MN, respectively) (9). An intraperitoneal insulin tolerance test (ITT) and a glucose tolerance test (GTT) were performed after a 12-h fast, using the method described previously (25).
Data are expressed as means ± SD (n = 3–10). Differences between two groups were assessed with Duncan's multiple range test, and differences among groups were evaluated by ANOVA. All statistical analysis was conducted using SPSS 6.1 software (SPSS Japan, Tokyo, Japan). Differences were considered significant at P < 0.05.
To examine the effects of Cbl-b deficiency on metabolic features of diet-induced obese mice, we compared food intake, wet weights of various tissues, and serum metabolic parameters in Cbl-b+/+ and Cbl-b−/− mice fed the ND or HFD for 5 weeks (Table 1). The concentrations of serum leptin and nonesterified FAs in Cbl-b−/− mice were significantly higher than those in Cbl-b+/+ mice fed the ND for 5 weeks. Body weight gain after being fed the HFD was higher in Cbl-b−/− mice than in Cbl-b+/+ mice, although the food intake was smaller in Cbl-b−/− mice than in Cbl-b+/+ mice. Consistent with this finding, serum leptin levels were higher in Cbl-b−/− mice fed the HFD than in Cbl-b+/+ mice fed the same diet. In contrast, serum adiponectin levels were lower in Cbl-b−/− mice fed the HFD. The serum levels of nonesterified FAs and triglycerides were higher in Cbl-b−/− mice fed the HFD than those in Cbl-b+/+ mice fed the same diet. These findings indicate that even under ND conditions, Cbl-b−/− mice had disordered lipid metabolism. The HFD also induced abnormalities of lipid metabolism in Cbl-b−/− mice.
Histochemical analysis showed marked recruitment of mononuclear cells into the WAT in Cbl-b−/− mice fed the HFD for 5 and 18 weeks compared with Cbl-b+/+ mice under the same conditions (Fig. 1A). Mean adipocyte size in the WAT of Cbl-b−/− mice fed the HFD for 5 weeks was increased compared with that of Cbl-b+/+ mice (Fig. 1B). Immunohistochemistry for F4/80, a marker of macrophages, indicated that these mononucleated cells were mainly macrophages (Fig. 1C). The number of crown-like structure (CLS) was increased in the WAT of Cbl-b−/− mice fed the HFD for 5 weeks compared with Cbl-b+/+ mice fed same diet (Fig. 1C). The expression of F4/80 was increased about twofold in the WAT of Cbl-b−/− mice compared with Cbl-b+/+ mice (Fig. 1D), in agreement with the results of immunohistochemical analysis (Fig. 1C). To further quantify macrophages in WAT, we performed flow cytometric analysis using cells in the adipose SV fraction obtained from Cbl-b+/+ and Cbl-b−/− mice fed the HFD for 5 weeks. Cbl-b deficiency was associated with an increased population of F4/80+-CD11b+ macrophages in the SV fraction (Fig. 1E). We also measured the expression of inflammatory cytokines and chemokines in the WAT of obese Cbl-b−/− mice. The HFD for 5 weeks increased the expression levels of IL-6 and TNF-α by ∼1.5-fold in Cbl-b−/− mice compared with Cbl-b+/+ mice (Fig. 1F). The WAT of obese Cbl-b−/− mice contained lower levels of adiponectin and higher levels of leptin compared with Cbl-b+/+ mice (Fig. 1G). The expression and serum levels of MCP-1 were higher in obese Cbl-b−/− mice than in Cbl-b+/+ mice (Fig. 1H and I).
Phosphorylation of Akt after insulin injection was attenuated in WAT, skeletal muscle, and liver of obese Cbl-b−/− mice, although insulin-induced Akt phosphorylation was noted in the respective organs of Cbl-b+/+ mice (Fig. 2A). We reported previously that elderly (>30-week-old) Cbl-b−/− mice fed the ND developed impaired insulin tolerance compared with normal insulin sensitivity in young (10-week-old) Cbl-b−/− mice fed the same diet (9). Consistent with the result of Akt phosphorylation, a significant impairment of insulin sensitivity was noted in young (12-week-old) obese Cbl-b−/− mice compared with their Cbl-b+/+ counterparts (Fig. 2B and C). In addition, the expression of suppressor of cytokine signaling-3 (SOCS-3), which interferes with insulin signal transduction (26), was increased in skeletal muscle and liver of Cbl-b−/− mice fed the HFD compared with Cbl-b+/+ mice fed the same diet (Fig. 2D). There were no differences in these parameters between Cbl-b+/+ and Cbl-b−/− mice fed the ND (data not shown). Because the expression of SOCS-3 is upregulated by proinflammatory cytokines, such as IL-6 (26), the easier induction of obesity in Cbl-b−/− mice may explain systemic insulin resistance via SOCS-3 expression.
Palmitate treatment significantly reduced the levels of Cbl-b mRNA and its protein in peritoneal Cbl-b+/+ macrophages and RAW264.7 cells, respectively (Supplementary Fig. 2), suggesting that dietary lipid can inactivate Cbl-b–mediated signaling in macrophages. To elucidate the role of Cbl-b in macrophage activation, Cbl-b−/− macrophages were treated with saturated FA. Palmitate upregulated IL-6 expression in Cbl-b+/+ macrophages, and Cbl-b gene deficiency further enhanced palmitate-mediated IL-6 expression in macrophages (Fig. 3A). In addition, treatment with this saturated FA enhanced IL-6 expression in macrophages but not in lymphocytes or neutrophils (Supplementary Fig. 3). We also examined the response of NF-κB and JNK signaling pathways, which are essential for IL-6 expression (27), in peritoneal Cbl-b+/+ and Cbl-b−/− macrophages treated with palmitate. Consistent with previous reports (13,28), incubation of peritoneal Cbl-b+/+ macrophages with palmitate stimulated IκB phosphorylation (Fig. 3B). The palmitate-stimulated IκB phosphorylation was significantly enhanced in peritoneal Cbl-b−/− macrophages (Fig. 3B). Interestingly, palmitate gradually degraded total IκB in Cbl-b−/− macrophages (Fig. 3B). Phosphorylation of the 32Ser residue of IκB causes its degradation (29). This finding supports the enhanced phosphorylation of IκB in Cbl-b−/− macrophages. Moreover, the palmitate-stimulated JNK phosphorylation, as well as IκB phosphorylation, was significantly enhanced in peritoneal Cbl-b−/− macrophages (Fig. 3C). These findings suggest that Cbl-b gene deficiency is associated with saturated FA-mediated activation of NF-κB and JNK signaling in macrophages. To elucidate this hypothesis, we also examined the effects of chemical inhibition of both pathways on IL-6 expression in peritoneal macrophages. Consistently, both NF-κB and JNK inhibitors significantly suppressed palmitate-induced IL-6 expression in Cbl-b−/− macrophages (Fig. 3D).
To determine whether Cbl-b is associated with the TLR4 signaling, peritoneal macrophages were treated with a neutralizing anti–TLR4/MD-2 IgG, which is known to suppress LPS-induced cytokine production (30). Blockade of TLR4 signaling with the neutralizing antibody suppressed the enhancement of palmitate-induced IL-6 expression observed in Cbl-b−/− macrophages (Fig. 3E). This result suggests that TLR4 expression in Cbl-b−/− macrophages was higher than that in Cbl-b+/+ macrophages, so that deficiency of the Cbl-b gene further enhanced the palmitate-enhanced IL-6 expression in macrophages. To dismiss this hypothesis, we examined palmitate-induced TLR4 expression in Cbl-b−/− macrophages. Throughout the experiment (until 16 h after treatment), there was no significant change in TLR4 expression in Cbl-b+/+ macrophages, whereas the levels of TLR4 transcripts in Cbl-b−/− macrophages before palmitate treatment were significantly lower than these in Cbl-b+/+ macrophages (Fig. 3F). The levels of TLR4 gradually increased after palmitate treatment and reached the levels of Cbl-b+/+ macrophages at 16 h (Fig. 3F).
We examined the effects of Cbl-b overexpresion on saturated FA-mediated expression of IL-6 in RAW264.7 macrophages. RAW264.7 macrophages transfected with the Cbl-b gene by lipofection expressed high amounts of Cbl-b protein, whereas the same cells transfected with the mock vector hardly expressed Cbl-b protein (Fig. 4A). As expected, overexpression of Cbl-b significantly suppressed palmitate-mediated IL-6 expression (Fig. 4B). The palmitate-induced changes in IκB phosphorylation and its protein level were suppressed by overexpression of Cbl-b in RAW264.7 macrophages (Fig. 4C). Palmitate stimulated IκB phosphorylation in mock vector-transfected RAW264.7 macrophages but gradually reduced its protein level. Overexpression of Cbl-b significantly suppressed palmitate-induced IκB phosphorylation and its degradation. Similarly, palmitate-induced JNK phosphorylation in mock vector-transfected RAW264.7 macrophages was canceled by overexpression of Cbl-b (Fig. 4D). These findings are consistent with the results shown in Fig. 3 and suggest that Cbl-b regulates saturated FA-mediated cytokine production in macrophages through the NF-κB and JNK signaling pathways.
To address whether Cbl-b plays a role in TLR4-mediated saturated FA signaling, HEK293/TLR4 cells transfected with mock vector or HA-tagged Cbl-b were treated with palmitate and assessed by immunoblotting for degradation of V5-tagged TLR4 and Myc-tagged MyD88 (Fig. 5A). Saturated FAs induced degradation of TLR4, but not MyD88, in the presence of Cbl-b (Fig. 5A). Vehicle treatment did not change the amount of TLR4 or MyD88 (data not shown). On the basis of these results, we also examined Cbl-b–mediated ubiquitination of TLR4. At 3 h after palmitate treatment, immunoprecipitates of V5-tagged TLR4 from Cbl-b–transfected cells, using anti-V5 antibody, contained large amounts of ubiquitinated TLR4 compared with those from mock-transfected cells (Fig. 5B). Furthermore, epoxomicin, an inhibitor of the proteasome, significantly blocked palmitate-induced TLR4 degradation (Fig. 5C).
Because immunoprecipitation and immunoblot analysis using commercially available TLR4 antibodies did not detect TLR4 protein in Cbl-b−/− macrophages, flow cytometry was performed to assess TLR4 protein levels on the surface of macrophages. The TLR4 protein expression level on the surface of peritoneal macrophages was similar in Cbl-b+/+ and Cbl-b−/− mice (Fig. 5D). Short-term (3-h) treatment with palmitate did not affect the expression of TLR4 (Fig. 3D) but significantly increased the TLR4 protein level on the surface of Cbl-b−/− macrophages. In contrast, the same treatment reduced the TLR4 protein level on the surface of Cbl-b+/+ macrophages. However, palmitate treatment did not induce degradation of MyD88 and TRIF, even in Cbl-b+/+ peritoneal macrophages (Fig. 5E). There were no differences in the protein levels of MyD88 and TRIF between Cbl-b+/+ and Cbl-b−/− peritoneal macrophages (Fig. 5E). These results indicate that the effects of FA on TLR4 protein level on the surface of macrophages are mediated by Cbl-b–induced ubiquitination and degradation.
BMT was conducted to elucidate the importance of Cbl-b in the hematopoietic system. Bone marrow cells derived from Cbl-b−/− (Cbl-b−/− BMT) or Cbl-b+/+ donors (Cbl-b+/+ BMT) were transplanted into Cbl-b+/+ recipient mice, followed by feeding the HFD to Cbl-b−/− BMT and Cbl-b+/+ BMT mice for 24 weeks. We measured serum parameters, glucose tolerance, homeostasis model assessment of insulin resistance (HOMA-IR), and cytokine expression in the WAT of Cbl-b+/+ and Cbl-b−/− BMT mice fed the HFD for 24 weeks. There were no differences in body composition and serum parameters between Cbl-b+/+ and Cbl-b−/− BMT mice fed the HFD for 24 weeks (Table 1). Furthermore, there was no significant difference in body weight gains after 24 weeks of the HFD (Fig. 6A). Because the body weight and epididymal fat weight were similar to those of aged-matched, nonradiated standard mice, Cbl-b−/− BMT mice were not considered to be obese. Although both groups of recipient mice fed the HFD for 12 weeks after BMT showed normal responses on ITT, the response on ITT was disturbed in Cbl-b−/− BMT mice fed the HFD for 24 weeks (Fig. 6B). In addition, glucose tolerance was exaggerated in Cbl-b−/− BMT mice compared with Cbl-b+/+ BMT mice (Fig. 6C). However, there was no significant difference in HOMA-IR between the two groups (Fig. 6D). In addition, being fed the HFD for 24 weeks significantly increased the population of F4/80+-CD11b+ macrophages in the WAT of Cbl-b−/− BMT mice compared with Cbl-b+/+ BMT mice (Fig. 6E). Furthermore, IL-6 and MCP-1 expression levels in the WAT of Cbl-b−/− BMT mice were higher than those in the WAT of Cbl-b+/+ BMT mice (Fig. 6F).
To confirm the involvement of Cbl-b in TLR4-mediated saturated FA signaling in vivo, Cbl-b+/+ and Cbl-b−/− mice fed the HFD for 5 weeks were intraperitoneally treated with Eritoran, a synthetic TLR4 antagonist. Eritoran did not change food intake, body weight, epididymal fat weight, or serum parameters, including blood glucose and serum insulin, of HFD-fed Cbl-b+/+ and Cbl-b−/− mice (Table 1). Consistent with increased inflammation in the WAT of Cbl-b−/− mice, being fed the HFD for 5 weeks significantly increased fasting blood glucose and serum insulin levels in Cbl-b−/− mice (Fig. 7A). Interestingly, Eritoran significantly reduced fasting blood glucose and serum insulin levels in Cbl-b−/− mice fed the HFD compared with placebo (Fig. 7A). Consequently, Eritoran significantly improved the impaired HOMA-IR in Cbl-b−/− mice fed the HFD (Fig. 7A). These results indicate that Eritoran improved insulin resistance specifically observed in Cbl-b−/− mice fed the HFD. In addition, Eritoran significantly suppressed the HFD-induced increase in serum IL-6 levels of Cbl-b−/− mice (Fig. 7B). Histochemical analysis of WAT also showed that blockade of TLR4 resulted in inhibition of macrophage infiltration into WAT and reduced CLS formation by infiltrated macrophages (Fig. 7C and D). On the basis of our previous finding that MCP-1 is a critical chemokine that can induce macrophage infiltration into WAT (9), we also measured MCP-1 expression in WAT in the current study. MCP-1 expression was higher in the WAT of Cbl-b−/− mice than in Cbl-b+/+ mice, and Eritoran treatment significantly suppressed the increased expression (Fig. 7E). As a result of these findings, we suggest that Eritoran reduces macrophage accumulation and CLS formation, at least in part, through the suppression of MCP-1 expression.
Long-term HFD feeding also induces obesity-associated insulin resistance, even in wild-type mice (5). Therefore, we tested whether Eritoran alters insulin resistance. The use of the HFD for 10 weeks induced hyperlipidemia and insulin resistance, even in wild-type mice (Supplementary Table 2 and Supplementary Fig. 4), whereas both pathological states were not noted in wild-type mice fed the HFD for 5 weeks (Table 1 and Fig. 7A). ITT results indicated that Eritoran treatment failed to improve the 10-week HFD-induced resistance in wild-type mice compared with placebo treatment (Supplementary Fig. 4). Eritoran also did not reduce HFD-induced accumulation of macrophages into the WAT in wild-type mice (data not shown), whereas feeding the HFD for 10 weeks resulted in accumulation of macrophages in the WAT in wild-type mice to an extent similar to that seen in wild-type mice fed the HFD for 18 weeks (Fig. 1A). In addition, Cbl-b expression in circulatory monocytes and immune cells, mainly macrophages, in the SV fraction, was not changed in wild-type mice (Supplementary Fig. 5).
Cbl-b functions as a negative regulator for activation of immune cells, such as lymphocytes and neutrophils, by ubiquitination of phosphatidylinositol 3-kinase and protein kinase C-θ (31,32). However, little information is available on the role of Cbl-b in the regulation of macrophages in contrast to its well-described roles in other immune cells (8,15,33,34). Interestingly, we found that palmitate treatment stimulated IL-6 expression in macrophages but not in lymphocytes or neutrophils (Supplementary Fig. 4). In this study, we investigated the function of Cbl-b in macrophages, especially as a negative regulator of FA-mediated cytokine expression in macrophages.
We reported previously that deficiency of the Cbl-b gene was associated with activation and recruitment of macrophages into WAT and caused peripheral insulin resistance in elderly mice (9), suggesting the involvement of Cbl-b in aging-induced insulin resistance. In addition to aging, obesity is another inducer of macrophage recruitment into WAT (4,5). To examine the relationship between saturated FAs and Cbl-b expression in macrophages, we treated peritoneal macrophages with palmitate. We found that palmitate treatment significantly downregulated Cbl-b expression in peritoneal macrophages and RAW264.7 cells at the protein and mRNA levels, respectively (Supplementary Fig. 4). Furthermore, increased saturated FAs can activate macrophages in WAT of Cbl-b−/− mice. Interestingly, recruitment of macrophages into WAT was noted in early stages of obesity in Cbl-b−/− mice. Moreover, insulin signaling was inhibited in insulin-sensitive organs of Cbl-b−/− mice fed the HFD compared with Cbl-b+/+ mice. These findings suggest that HFD- and aging-induced recruitment of macrophages into WAT, as well as insulin resistance, are at least partly mediated by Cbl-b.
Downregulation of Cbl-b expression accentuates sepsis-related production of proinflammatory cytokines and chemokines in a model of polymicrobial acute pneumonia associated with TLR4 (15). Thus, Cbl-b can regulate the sepsis-induced TLR4-mediated acute inflammatory response. We report here that Cbl-b deficiency increased the expression of cytokines in macrophages induced by saturated FAs and that saturated FAs induced ubiquitination and degradation of TLR4 in the presence of Cbl-b. These findings suggest that Cbl-b negatively regulates saturated FA-mediated TLR4 signaling through ubiquitination of TLR4. Because Cbl-b recognizes the site of tyrosine phosphorylation in target protein receptors, followed by downregulation by ubiquitination (35), the recent evidence that LPS activates TLR4 through phosphorylation of 674Tyr and 680Tyr of human TLR4 (36) also supports our hypothesis. The current study also showed that saturated FAs served as ligands for TLR4 to induce the activation of NF-κB and JNK in Cbl-b−/− macrophages. This finding is consistent with the evidence that Cbl-b controls the link between TLR4 and intracellular adaptor MyD88 and regulates NF-κB cytokine signaling (15). In contrast, Han et al. (37) reported recently, both in vitro and in vivo, that Cbl-b negatively regulated LPS-induced TLR4 activation by ubiquitination and degradation of MyD88 and TRIF, but not TLR4. We found that palmitate treatment did not induce degradation of MyD88 and TRIF even in Cbl-b+/+ peritoneal macrophages. FAs may induce TLR4 activation through a mechanism different from that of LPS. The reason for this discrepancy remains unknown. Further studies are necessary to elucidate such mechanism.
We used whole-body knockout mice of the Cbl-b gene in the current study. Therefore, we performed BMT to confirm the importance of Cbl-b in the hematopoietic system. The bone marrow-derived cells with specific depletion of Cbl-b gene exaggerated HFD-induced abnormalities on ITT and GTT, whereas a longer time after BMT elapsed before the infiltration of macrophages into the WAT of mice fed the HFD. Body weight was not different between mice transplanted with Cbl-b+/+ and Cbl-b−/− bone marrow cells. Consistent with our findings, BMT of TLR4 gene-depleted cells did not induce changes in body weight but did ameliorate HFD-induced inflammation and insulin resistance in mice (19). In addition, palmitate-induced overexpression of IL-6 was noted only in macrophages, but not in lymphocytes or neutrophils (Fig. 3B and Supplementary Fig. 5). On the basis of these results, we suggest that Cbl-b plays a critical role in hematopoietic cells, especially in macrophages, in mice.
FA treatment preferentially downregulated the Cbl-b protein level in macrophages in vitro, leading to their activation. Interestingly, Cbl-b−/− BMT mice fed the HFD for 24 weeks showed disturbed responses in the ITT and GTT, accompanied by infiltration of macrophages into WAT, regardless of the nonobese status. Profound hyperlipidemia and insulin resistance without obesity were described recently in humans and in an animal model of lipodystrophy (38,39). Severe hyperlipidemia may induce activation and infiltration of macrophages into WAT, possibly through downregulation of Cbl-b levels. Obesity is not necessary for macrophage-mediated insulin resistance, although the HFD tends to induce obesity. However, the results from the BMT experiments could not exclude the possibility that the adiposity observed in Cbl-b−/− mice fed the HFD is associated with insulin resistance. Therefore, insulin resistance in Cbl-b−/− mice fed the HFD accompanied with adiposity was more apparent than that in Cbl-b−/− BMT mice fed the HFD. Further examination is necessary to elucidate this hypothesis.
Treatment with Eritoran failed to improve the 10-week HFD-induced resistance in wild-type mice (Supplementary Fig. 4). Feeding the HFD for 10 weeks did not change Cbl-b expression in circulatory monocytes and adipose tissue macrophages, although it induced hyperlipidemia in these mice (Supplementary Table 2 and Supplementary Fig. 5). These findings indicate that feeding the HFD for 10 weeks does not activate TLR4 signaling in wild-type mice. Presumably, severe inflammation, such as macrophage activation associated with Cbl-b deficiency, did not occur in the WAT of wild-type mice fed the HFD for 10 weeks. These findings are consistent with the previous report that the initial stage of HFD-induced insulin resistance is independent of inflammation, whereas the more chronic state of insulin resistance is mediated by macrophage-induced proinflammatory action (40).
TLR4-deficient mice and C3H/HeJ mice, which carry a loss-of-function mutation in TLR4, are protected against HFD-induced insulin resistance and suppress saturated FAs-induced overexpression of cytokines (13,14,28). In this study, we examined the effect of a chemical analog, Eritoran, on insulin resistance and IL-6 expression in vivo. Eritoran is a structural analog of the lipid A portion of LPS and binds with TLR4 as an antagonist, and the crystal structure of Eritoran bound to the TLR4-MD2 complex has been described (18). Indeed, Eritoran inhibits LPS-induced TNF-α production in human whole blood samples (41). Interestingly, Eritoran inhibited the production of proinflammatory cytokines in macrophages stimulated by FA-induced TLR4 activation and also improved HFD-induced insulin resistance and the serum IL-6 level in Cbl-b−/− mice. In addition, we found that Eritoran treatment decreased CLS formation in WAT of Cbl-b−/− mice fed the HFD, at least in part, through reduction of MCP-1 expression in WAT. As described above, activation of Cbl-b–mediated ubiquitination of TLR4 is effective in inhibiting obesity-induced expression of proinflammatory cytokines. Taken together, inhibition of TLR4 signaling by Eritoran or Cbl-b–mediated degradation seems an attractive therapeutic strategy for the treatment of obesity-related insulin resistance.
fn1This article contains Supplementary Data online at http://diabetes.diabetesjournals.org/lookup/suppl/doi:10.2337/db12-0677/-/DC1.
This study was carried out as part of the Ground Research Announcement for Space Utilization, promoted by the Japan Aerospace Exploration Agency and the Japan Space Forum, to T.N. The study was supported by a grant-in-aid for scientific research from the Ministry of Education, Culture, Sports, Science, and Technology of Japan to T.N., Seed Production by the Japan Science and Technology Agency to T.N., and the Promotion of Basic Research Activities for Innovative Biosciences from the Bio-Oriented Technology Research Advancement Institution of Japan to T.N. This study was also partly supported by a high-technology research center grant from the Ministry of Education, Culture, Sports, Science, and Technology of Japan to the Juntendo University Research Institute for Disease of Old Ages, Tokyo, Japan.
No potential conflicts of interest relevant to this article were reported.
T.A. and K.H. designed the research; researched the data, including additional data for revision; and wrote the manuscript. S.Ka., S.Ko., A.Oc., K.U., A.S., and J.T. researched the data, including additional data for revision. A.Oh. and Y.M. contributed to the new analytic tools. S.T.-K. and Y.O. contributed to the new analytic tools and reviewed and edited the manuscript. M.O. and E.M.M. reviewed and edited the manuscript. T.N. designed the research, reviewed and edited the manuscript, and wrote the manuscript. T.N. is the guarantor of this work, and, as such, had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
The authors thank Stanley Lipkowitz (from the U.S. National Institutes of Health) for pCEFL-Cbl-b-HA, and Toshiaki Suzuki (from Tokyo Metropolitan Institute of Medical Science) for pcDNA3-FLAG-Ubiquitin, and are grateful to Hua Gu (from University of Columbia) and Tetsuya Inazu (from University of Ritsumeikan) for providing Cbl-b−/− mice. The authors thank Eisai Research Institute for providing E5564.
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